1. Field of the Invention
The present invention relates to a GMR device of the CPP structure for reading the magnetic field strength of a magnetic recording medium or the like as signals, a thin-film magnetic head comprising that device, and a head gimbal assembly and a magnetic disk system comprising that thin-film magnetic head.
2. Explanation of the Prior Art
With recent improvements in the plane recording density of hard disk systems, there has been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has gained great popularity, which has a structure wherein a reproducing head having a read-only magneto-resistive effect device (hereinafter often referred to as the MR device for short) and a recording head having a write-only induction type magnetic device are stacked together on a substrate.
For the MR device, there is the mention of an AMR device harnessing an anisotropic magneto-resistive effect, a GMR device harnessing a giant magneto-resistive effect, a TMR device harnessing a tunnel-type magneto-resistive effect, and so on.
The reproducing head is required to have high sensitivity and high outputs in particular. GMR heads using a spin valve type GMR device have already been mass-produced as a reproduction head possessing such performances, and to meet further improvements in plane recording densities, reproducing heads using TMR devices are now being mass-produced, too.
In general, the spin valve type GMR device comprises a nonmagnetic layer, a free layer formed on one surface of that nonmagnetic layer, a fixed magnetization layer formed on another surface of the nonmagnetic layer, and a pinning layer (generally an antiferromagnetic layer) on the side of the fixed magnetization layer facing away from the non-magnetic layer. The free layer has its magnetization direction changing depending on an external signal magnetic field, and the fixed magnetization layer has its magnetization direction fixed by a magnetic field from the pinning layer (antiferromagnetic layer). The fixed magnetization layer has, in its preferable aspect, a synthetic pinned layer comprising a nonmagnetic intermediate layer sandwiched between an inner layer and an outer layer.
Common GMR heads used so far in the art have a CIP (current in plane) structure wherein a current for detecting magnetic signals (the so-called sense current) is passed parallel with the plane of each of the layers forming the GMR device. On the other hand, GMR devices having the so-called CPP (current perpendicular to plane) structure wherein the sense current is passed perpendicularly to the plane of each of the layers forming the GMR device (that will often be called the CPP-GMR device for short), too, are now under development as next-generation ones. Note here that the aforesaid TMR devices, too, would come under the wide category of CPP structures.
The CPP-GMR device is expected to have ever higher potentials for the reasons that it is lower in resistance than the CPP-TMR device, and higher in output at a narrow track width than the CIP (current in plane)-GRM device.
The CPP-GMR device comprises, in its basic structure, a spin valve multilayer film wherein the antiferromagnetic layer acting as a pinning layer, the fixed magnetization layer as a pinned layer, the nonmagnetic spacer layer and the free layer are stacked together in order. And a pair of opposed shield layers are formed in such a way as to sandwich between them that spin valve multilayer film vertically in the stacking direction. Usually, that pair of shield layers are designed to function also as electrodes for passing the sense current in the stacking direction.
And now, for the fixed magnetization layer (pinned layer) of the spin valve multilayer film, there is a synthetic pinned layer used, wherein ferromagnetic layers are stacked together with a nonmagnetic intermediate layer of Ru, Rh or the like held between them. In this arrangement wherein the two ferromagnetic layers with the nonmagnetic intermediate layer held between them are anti-ferromagnetically coupled to each other, the magnetization of the fixed magnetization layer (pinned layer) is held back and stabilized because they remain mutually anti-parallel. When the spin valve multilayer film is used as a head's read device, it is possible to get around a displacement of the bias point due to a magnetostatic field from the fixed magnetization layer (pinned layer).
For such a reason, the magnetic moments of the two ferromagnetic layers forming part of the synthetic pinned layer must be well balanced. As this balance is thrown off, it will give rise to an increase in the net moment of either one of the two ferromagnetic layers, causing magnetization to become unstable against an external magnetic field. Of the two ferromagnetic layers forming part of the synthetic pinned layer, one (pinned layer) nearer to the nonmagnetic spacer layer will be called the inner pin layer, whereas one (pin layer) far away from it will be called the outer pin layer. It is the inner pin layer that contributes to the magneto-resistive effect; the outer pin layer serves to exclusively stabilize the magnetization of the pinned layer.
With the CPP-GMR device, by the way, there is the bulk scattering effect contributing much to the magneto-resistive effect. In other words, in the CIP-GMR device for comparison, the direction (conduction) of sense current flow lies in the film plane so that there is plenty of resistance change obtainable due to spin dependent scattering at the interface. With the CPP-GMR device, on the other hand, the sense current flows perpendicularly to the film plane, viz., the interface; that is, it passes through the interface, contributing less to that magneto-resistive effect. Further, because an ordinary GMR film has as little as two interfaces: the upper and lower planes of the nonmagnetic spacer layer, the contribution of the interfaces is particularly limited. For this reason, the magnetic layer should preferably be thicker to obtain high MR change rates.
In the meantime, the inter-shield gap must be as narrow as possible to achieve higher recording densities; the fixed magnetization layer (pinned layer) should preferably be thin. From such a point of view, if the thickness of the outer pin layer that makes no contribution to the magneto-resistive effect can be thin without sacrificing the thickness of the inner pin layer that contributes to the magneto-resistive effect, it would then be possible to make the inter-shield gap narrow while the resistance change is kept as such.
The situations being like this, an object of the invention is to provide a GMR device of the CPP structure using the synthetic pinned layer as the fixed magnetization layer (pinned layer), wherein the thickness of the outer pin layer is reduced at no cost of the thickness of the inner pin layer forming a part of the synthetic pinned layer and without doing damage to the function of the synthetic pinned layer per se, viz., resistance to an external magnetic field, thereby achieving higher recording densities.